The present invention relates to a silica glass composition and, more specifically, it relates to a silica glass composition, in which the lasing species is thulium, eg. the ion Tm3+. The invention has application in the field of waveguides.
Currently one strategy for increasing the amount of data transmitted across a telecommunications network is wavelength division multiplexing, WDM. In this scheme each wavelength channel is typically assigned a frequency slot with a bandwidth of 100 GHz (0.8 nm) in which to operate, and a large number of these channels are then multiplexed together and sent over a single transmission fibre. For transmission systems of less than 100 km the range of wavelengths over which signals can be transmitted extends from around 1430 to 1680 nm. In other words this range is only limited by the variation in the basic transmission properties of the optical fibre medium. However for systems of more than 100 km the attenuation in the optical fibre becomes significant such that the signal needs to be periodically boosted using an optical amplifier. Since the amplifier of choice for current systems is based on erbium doped silica fibre which has a bandwidth of about 30-40 nm this leads to strict limitations in the number of channels that can be effectively multiplexed onto a single fibre. Although some amplifier designs do allow the gain bandwidth to be increased by about a factor of 2 they are much less reliable due to the much higher component count, and are typically very costly. In any case, the 80 nm achieved with increased bandwidth amplifiers still only represents a small fraction of the optical fibres available bandwidth.
In order to increase the bandwidth of photonic amplifiers, rare earth elements other than erbium have been looked at. Thulium is one such rare earth element that has recently been the subject of some investigations.
W. S. Brocklesby, J. E. Townsend, D. J. B. Brinck, and R. S. Brown xe2x80x9cNon-radiative Relaxation in Ta-doped Silica Fibrexe2x80x9d, Opt. Mats, Vol 3. pp205-208, 1994 describe silica glasses doped with the Tm3+ ion at a doping concentration of approximately 75 parts per million (ppm). Some of the properties thulium doped tantalosilicate fibres, thulium doped germanosilicate and thulium doped aluminosilicate fibres have been investigated. In particular Brocklesby et al have observed that the fluorescence decay components in thulium doped tantalosilicate glasses are longer than those in thulium doped germanosilicate or thulium doped aluminosilicate fibres. Although, the addition of tantalum to the silica glass would appear to increase the fluorescence decay time in the thulium doped tantalosilicate fibre, the fibre suffers large losses which rule out its use as an optical amplifying medium.
The present invention seeks to provide to provide a thulium doped fibre having increased fluorescence decay times and reduced loss (i.e reduced attenuation of optical signals).
The invention, which is more closely defined by the claims 1-25, is based on the surprising discovery that the presence of germanium in low concentrations in a thulium doped tantalosilicate glass improves the efficiency of the glass, because it decreases the attenuation of optical signals.